Best option to improve bathroom extraction

4 Dec 2011
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United Kingdom
We inherited my late in-laws house. Among other things, we added gas central heating and a powerful over bath shower. The bathroom is 2.5m x 2.3m so just under 6 square metres. We have a heated towel rail in there and a 100mm Airflow Aventa in-line fan (rated 52 litres per second) ducting out of a soffit through no more than 2.5m of flexible ducting. Despite this the room is like a sauna after a shower resulting in condensation and mould. The insulation above the bathroom in the loft is not brilliant and I am going to look at fixing that but doubt it would make a massive difference.

Having the bathroom door open and window open (not ideal) makes only a small difference. Even when the heated towel rail has been on for a while the room can’t really be described as toasty.

So looking at options to try and improve things. The simplest option would be replacing the fan with another more powerful 100mm fan - the Manrose 100 claims to shift 30% more than the Airflow. The existing grill is above the toilet about 120cm from the shower so maybe moving that closer to the shower might help? Next option would be a 125mm or 150mm in-line fan which would have significantly higher extract rates but would necessitate replacing the ducting.

I guess the ideal solution would be to get more heat in the room but not sure of the best way of doing that.

Would appreciate the thoughts of people more experienced than me.

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The path the airflow takes can make a big difference. If your extractor isn't over the shower then it's possible that the fresh air that comes in under the door cuts past the shower and goes straight out the extractor fan.

I'd be looking at moving the extractor fan.

Insulation is always nice and might help a little on condensation, but fundamentally you've got to get the damp air out. That's all about airflow, the insulation just gives you a bit more time to do it.

If your fan doesn't have an overrun timer then you should swap it out for one that does. It won't help the sauna like nature of your showers but it will help get the remaining damp air out once you're done.
It’s spooky you mention about the airflow. I was reading about that earlier. And yes the most direct route between the door and extractor bypasses the shower. It does have an overrun so that part’s fine. I will have a go at moving the extraction point - thanks.
I've done this plan of the bathroom. Original scale is 1:250 - no idea if posting here resizes it though - but should still be in proportion!
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for steamy, the airflow direction is less critical, because water vapour is lighter than air, so it will rise towards the ceiling, where the fan will catch it.

A more powerful fan, with a run-on timer, is I think the first thing you need.

I work in cubic metres of air per hour. a typical cheap bathroom fan will shift about 80 c.m/hr but if your duct is in the loft and you can get at it, you can have three times that.

If your existing fan is the AV100, it is already fairly powerful, at 187 cu.m/hr, so check that your fan, grille and duct are not choked with fluff, cobwebs and birds nests. Straight rigid duct is cleaner than the convoluted hose. Tilt it so any condensation runs outside and cannot collect in the duct. You need a half-inch gap under the bathroom door to let air in.

here is an example of a good quality 250 cu.m/hr fan (it is also very quiet). Cheaper ones are also available, but check their throughput and noise rating. 25db(A) is pretty quiet.

Start it with the timer set to max (usually 20 minutes). If you wish, you can reduce the run-on; but with a very quiet fan, it wil not matter. For extra silence, mount the fan on a sheet of thick ply, with carpet or underlay beneath it, screwed to the loft joists.
Thanks John but our current fan extracts at 52 l/s compared to 31 l/s of the Manrose one you mentioned. Manrose do make a more powerful one at 68 l/s. Timer is already set to maximum.
yes, I realised that later and have now corrected my post.

BTW see if yours is a 2-speed fan and has been set to "low."

When I worked out the cu.m/hr rating, I was a bit surprised it was not doing better.
Hi John

Great minds.... that was the first thing I checked and was hoping it was set to ‘low’. Nope - set to high.
Having the bathroom door open and window open

You definitely don’t want the window open when the fan is running.
Ideally the door would have a small gap underneath, or a vent. You don’t want it to be fully open, nor hermetically sealed.

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